Who’s digging in my lawn?
Residents of south-eastern Australia, especially of the Dandenong Ranges near Melbourne, may find round holes appearing on their lawns. Sometimes if left alone high chimneys of soil grow up from the holes. These structures are the work of land crayfish.
Who digs the burrows?
Land crayfish (sometimes called “land yabbies” or “land crabs” although they are not crabs) are relatives of yabbies, Murray crayfish and, more distantly, marine scampi. This group of Crustacea, the Astacidea, is recognised by the possession of a pair of equal large claws and two other pairs of smaller nippers. That is, three of the five pairs of legs are chelate.
The family Parastacidae, to which yabbies and land crayfish belong, is found only on southern continents but is most diverse in Australia. Land crayfish are one of 34 species in the genus Engaeus. Many of these species have a quite limited geographic range and some confined only to a single catchment.
Species of Engaeus are smaller than the more familiar and edible yabbies (Cherax destructor) and only grow to a maximum length of 120 mm. The carapace is more flattened from side to side than in a yabby and the abdomen (tail) is smaller and often bristly. So small in fact that they are inedible.
What is under the openings of the burrows?
Land crayfish live only in damp environments, which is why the hilly regions of eastern Victoria and Tasmania are home to most species. Here, they are found in swamps, near streams, eucalypt forests or rainforests. Their subterranean homes vary according to species and depend on their habitat.
Some species, such as Engaeus urostrictus, live close to streams. Here they build burrows down to 25 cm deep at the level of the water table. Water fills a small chamber at the bottom of the branching burrowing system covering half a square metre. As the crayfish excavates the burrow it brings soil to the surface and places the pellets around the entrance until a chimney grows up to 13 cm high.
Other species, Engaeus tuberculatus for example, live on slopes away from streams. Their burrows do not reach the water table and the subterranean chambers rely on rain or seepage to fill with water. Exit burrows diverge near the surface where there are several openings. Waste from the excavations is brought to the surface where pellets of soil are deposited and run down the slope to form a fan of dirt.
What are the crayfish doing down there?
The crayfish spends most of its time in the chamber at the bottom of its burrow so is rarely seen. Only at night does it become active bringing soil to the surface. It feeds mostly on decaying roots and buried plant matter, but sometimes eats worms or insects. Some species live alone, but others are more communal.
Male and females usually come together to mate in spring and eggs are incubated attached to the tail of the female. Juveniles hatch in mid–late summer. After hatching they may remain in their parents’ burrow or migrate to start a life of their own. Living amongst the crayfish is a very specialised fauna of aquatic crustaceans and insects, some not found in other watery habitats.
Because the burrowing crayfish’s habitat is threatened by land clearing and drainage, several Engaeus species are considered threatened or vulnerable.
Horwitz, P. H. J., Richardson, A. M. M. and Boulton, A., 1985. The burrow habit of two sympatric species of land crayfish, Engaeus urostrictus and E. tuberculatus (Decapoda: Parastacidae). Victorian Naturalist 102: 188–197.